These programs trade in millionths of a second, millions of trades a day, and are even trained to prey on other algorithms by recognizing their patterns, stealing their stock picks, and selling them back to them. They even get names, like, "Dagger," "Barcode" or "Crystal Triangle," that apparently come from the Drug Dealer Brand Name Generator.
Malfunctioning algos have shut down the London Stock Exchange and briefly paralyzed the New York Stock Exchange among others. In one incident, a guy not double-clicking fast enough triggered a bug in the program.
When you're controlling billions of dollars, you don't have time to use an antiquated semi-automatic mouse.
Instead of slowing down to work out the bugs, finance companies keep rolling out bigger and more aggressive algorithms. The bank that crashed the NYSE didn't even know they'd done it until they got a (probably very angry) phone call the next day.